1. Field of the Invention
The present invention relates to an image processing method and device for moving images and, more particularly, to an image processing method and device both of which continuously perform resolution conversion.
2. Description of the Related Art
A large number of resolution conversion techniques for still images have already been proposed. As general methods, there are image interpolation methods such as a nearest neighbor method, a bi-linear method and a bi-cubic method as well as frequency range interpolation methods such as DCT coefficient interpolation.
In general, resolution conversion of a moving image which is a continuation of still images is also performed by a method similar to any of the above-mentioned ones.
This resolution conversion needs to use a method such as a bi-cubic method which requires a large operation amount, in order to obtain a high-quality result. Such a method does not become a problem as long as it is executed on dedicated hardware, but if the method is executed by software processed by a general-purpose processor, the operation amount may become a problem.
Moving images are generally compressed by means of standards such as MPEG2 and MPEG4 because of other problems such as the problem of the capacity of a transmission path or a storage medium. These standards also require processing such as Huffman decoding or IDTC which consumes a large operation amount, and if decoding processing is performed by software, the operation amount may become a problem.
It has heretofore been general practice to perform the decoding processing and resolution conversion processing of compressed moving images by dedicated hardware because of the problem of the operation amount, but the number of cases where such processing is performed by software has increased with the recent improvement of processing technology.
When consideration is given to the case where software is used to perform decoding processing and further resolution conversion on a compressed moving image such as an MPEG image by software, the above-mentioned throughput problem becomes more remarkable. Namely, the calculations of both resolution conversion and decoding processing must be in time for real time display. However, the calculation amount necessary for the software decoding processing of pictures of MPEG generally differs for each of the pictures, so that there occurs the problem that the calculation capability of a processor becomes insufficient to perform accurate resolution conversion of a picture which consumes a large decoding calculation amount.
Accordingly, when decoding processing and resolution conversion of a moving image are to be performed by software, it is general practice to take into account a calculation amount for the decoding processing and perform simple resolution conversion which can be processed within the processing capability of a processor. However, in this method, it is difficult to use resolution conversion requiring a large calculation amount, and it is impossible to expect high image quality.
An invention which takes note of the calculation amount of decoding processing of moving images is disclosed in JP-A-5-122681. In this invention, if decoding processing exceeds the processing capability of a processor, the decoding processing is switched to a simple decoding method which can reduce a calculation amount while decreasing resolution. When a solution to the above-mentioned problems is considered from this invention, it is possible to readily presume a method which dynamically monitors the calculation amount of a system and, if the decoding calculation amount is too large to perform real-time processing with a resolution conversion method which is in use, switches the resolution conversion method to a simple resolution conversion method using a smaller calculation amount. However, this method has the problem that when the capability of a processor becomes insufficient, the resolution conversion processing methods are abruptly switched, so that there occurs the problem that the degradation of image quality becomes large.